1. Field of the invention
The present invention relates generally to nonvolatile memory devices using resistive elements and an associated driving method.
2. Discussion of Related Art
Next generation nonvolatile memory devices are being developed for use in portable consumer products to provide high capacity and low power consumption. While dynamic RAM (DRAM) and flash memory devices store data by using charge, nonvolatile memory devices utilize resistance material to store data by changing the state of phase-change material. These memory devices include, for example, PRAMs (Phase change Random Access Memory) utilizing phase-change material such as a chalcogenide alloy that can be switched between two states, RRAMs (Resistance Random Access Memory) employing material having a variable resistance characteristic of complex metal oxides, and MRAMs (Magnetic Random Access Memory) utilizing the resistance change of MTJ (Magnetic Tunnel Junction) thin films according to the magnetization state of a ferromagnetic substance. The resistance value is maintained in these devices even when no current or voltage is supplied demonstrating nonvolatile memory characteristics.
In a phase-change memory cell, when the material is heated and then cooled, the phase-change material transforms into a crystalline state or an amorphous state. The material has a low resistance in the crystalline state and a high resistance in the amorphous state. The crystalline state may be defined as data “set” or data “0,” and the amorphous state may be defined as data “reset” or data “1.” In order to write data on the phase-change memory cell, the state of the phase-change material must be changed by a sufficiently high write current. In order to read data from the phase-change memory cell, a read current (or sensing current) smaller than the write current must be provided to maintain the state of the phase-change material.